Advancement of informatization allows people to communicate at any moment and everywhere. Extensive use of various communication equipments such as handsets, blue tooth earphone and stereo headphone and communication technologies greatly facilitate people's life and increase work efficiency. However, the social development results in a severe issue, that is, noises. In a noisy environment, definition and intelligibility of communication voice are severely compromised and when noise is high to a certain degree, communication can not proceed, and people's audition and physical and mental health will be hurt.
For communication under strong noise background, prior art solution conducts speech enhancement and noise reduction processing in terms of the following aspects: one the one hand, acoustics signal processing technologies are applied at the sending end of the communication equipment to increase SNR of speech signals picked by the microphone, allowing a remote user to hear proximal user's talking; on the other hand, it is required to enhance SNR of speech at receiving end of the communication equipment, allowing proximal end to hear speech signal sent by remote user clearly.
However, increasing SNR of speech at receiving end has always been a difficulty in the art. In order to increase SNR of speech at the receiving end, two methods have been proposed in prior art.
One method is to apply the automatic volume control technology (see China Patent Application Publication CN1507293A) in which the power delivered to speaker unit is automatically enhanced when there is much noise in the surroundings. This is a passive noise reduction processing method, and due to limitation by the power of speaker unit itself and the sound pressure fed into ear, the volume of speaker unit can not be enhanced without indefinitely. Furthermore, high intensity voice emitted by the speaker may injure user's audition and physical and mental health. Therefore, this noise reduction processing method has limited speech enhancement.
Another method is to apply traditional combined active/passive noise control technology (see China Patent Application Publications CN101432798A and CN101001481A) to a closed communication earphone. This closed earphone is classified into head-worn type and earplug type. Closed earphones typically use a structure and materials which hermetically couple with ears. In this type of closed earphones, intermediate and high frequency noise is reduced through sound absorption and isolation by the materials of the earphones, low frequency (mainly below 500 Hz) noise is effectively reduced by active noise control technology, thereby realizing sufficient noise cancellation in full band and effectively enhancing speech SNR of earphones at receiving end. However, wearing closed communication earphones for a long time makes the user feel unbalanced air-pressure between inside and outside of the ear canal. Discomfort caused by wearing this kind of earphones is a major factor that impairs the active noise reduction technique of this configuration from widespread use in communication equipments.
It is a highly demanding and yet challenging topic to realize feed-forward active noise cancellation technology to enhance sound SNR at the receiving end while ensuring communication equipments such as handsets, blue tooth earphones and stereo headphones have non-closed structures.
FIG. 1 is a schematic diagram of traditional noise cancellation at the receiving end of a communication equipment using a non-closed feed-forward active noise control technology. As shown in FIG. 1, the implementation of feed-forward active noise control system is based on the hypotheses that surrounding noise firstly propagates to the microphone and then to ears. In the case the noise propagates to the microphone 102, the propagation path will be divided into two channels. The first channel is along the acoustics channel P shown in FIG. 1 to propagate to ears in physics space, as shown in FIG. 1 by solid lines, wherein P is an acoustics transfer function of surrounding noise propagating from the microphone to ears. Another channel extends through the electronic circuits, as shown in FIG. 1 as a transport path generating anti noise from microphone 102, through speaker 104 and to ears. As shown in FIG. 1 by the broken line, it may be expressed by H and G connected in series, wherein H is the frequency response of the active noise reduction circuit and G is the transfer function from speaker to ears, which is called a secondary channel. Assuming that it is designed as P=−GH on the noise reduction frequency band, namely P and GH have same amplitude and opposite phases, then the original noise and antinoise propagating via the two channels respectively superimpose to be balanced out at ears, thereby realizing noise reduction.
In order to design the acoustics channel P, a frequency response of the active noise reduction circuits and the transfer function G from speaker to ears to meet the requirement, P=−GH, it generally needs to design and process the front and rear chamber of the speaker, e.g., to adjust the dimension of the front and rear chamber and the size of the opening, so as to change the transfer function G from speaker unit, so that noise is complete canceled upon one noise cancellation process.
The major problem in implementation of communication systems that adopt non-closed feed-forward noise cancellation is that the acoustics channel P and the secondary channel G may vary depending on coupling state of communication equipments and ears. With the frequency response H of the circuit part remaining constant, noise reduction performance realized when different people use it or the same people use it at different times are inconsistent, that is, some times noise reduction performance is good, sometimes deteriorated, or even no noise reduction effect is felt at all.
For the purpose of comfort when wearing earphones and consistency in noise reduction, China Patent Application Publication CN101432798A proposes a technical solution for improving the structure of earphones, in which, the coupling between earphones and ears are adjusted so as to achieve comfort and consistency in noise reduction. FIG. 2a illustrates a possible structure of the in-ear portion of a non-closed earphone that penetrates the ear, which has a small tapered end portion penetrating into the ear. By making the small tapered end portion penetrate into the ear, the propagating path become shortened between the speaker and the human ear, so that it guarantees that the acoustics transfer function for different people wearing the earphones has excellent consistency. FIG. 2b illustrates another technical solution for improving the structure of earphones. In the acoustics structure shown in FIG. 2b, two sound-passing grooves are disposed at the upper and lower portions of the in-ear portion of the non-closed earphone, so as to ensure a certain degree of sound leakage whether the earphones are tightly or loosely worn or no matter the earphones are coupled with ears of different sizes. Therefore, the structure shown in FIG. 2b not only ensure comfortability but also excellent consistency of P function and G function for different people wearing the earphone. Yet the solutions illustrated in FIGS. 2a and 2b achieve the consistency in noise reduction by changing the structure of an earphone, though the change in structure makes some progress, it can not fundamentally solve the problem of in-consistency in noise reduction effect applying the non-closed forward-feed noise reduction technology to a communication equipment.
Methods for addressing this inconsistent noise reduction performance are at present mainly self adapting active noise cancellation by DSP (digital signal processing), but there are two limitations for this technology to be applied to communication equipments such as handsets, non-closed blue tooth, stereo headphones. On one hand, the feed-forward adaptive noise canceling algorithm uses FX-LMS algorithm, which needs to recognize the secondary channel G to obtain Ĝ. The error in recognition of the secondary channel will influence system stability directly, and the secondary channel G itself may vary greatly during operation of the above-mentioned non-closed communication system, therefore it is difficult to guarantee algorithm stability. On the other hand, for communication equipments such as handsets, non-closed blue tooth and stereo headphones, due to the volume limitation of equipments themselves, the time delay of acoustics channel P is very small, if DSP is used for adaptive noise cancellation, there is a very high requirements on sampling rate of the system, and the system power consumption and frequency band for noise reduction are both limited greatly.